The relationship between force (P) and velocity (V) of shortening will be investigated using skinned single muscle fibers of the frog (i.g., fibers whose membrane has been mechanically removed) whereby the behavior of the contractile apparatus can be studied under controlled conditions, obviating the complications of excitation and excitation-contraction coupling. The influence upon the P-V curve of different degrees of calcium activation and of different initial sarcomere lengths will be studied in solutions containing creatine phosphate and creatine phosphokinase to ensure that ATP levels remain high throughout the fiber cross-section. The influence of varying degrees of calcium activation will test whether activating calcium merely turns cross-bridges on and off or whether it has a modulating influence as well upon the kinetics of cross-bridges which it has turned on.